Conventional infrared photodetectors formed of cadmium mercury telluride (CMT) alloy materials are well known, both simple two contact photoconductors and single p-n homojunction photodiodes. Recent developements have seen time-delay-integration functions included in the photoconductive element--see for example UK Pat. No. 1,488,258, where the detector is of strip shape and the photocarrier drift rate is matched to the velocity of a scanned image.
A major problem associated with conventional photoconductors, particularly those used in unscanned "staring" applications is a standing DC output (known as a bias "pedestal") even in the absence of any input optical flux. For conventional long wavelength intrinsic photoconductors, the impedance is low, and the standing current is typically several milliamps corresponding to a pedestal of a few volts. For comparison the background flux signal is millivolts, and the required optical signal is microvolts. This pedestal is exceptionally difficult to subtract out in a stable and accurate manner except in the case of a scanned system which can use capacitative output coupling.
Photovoltaic detectors--e.g. photodiodes, have the advantage that they do not require bias, do not dissipate significant power, nor do they have a bias pedestal. Juction diode detectors, however, are difficult to make and require a bulk of scarce and largely uncharacterized p-type CMT. They also suffer stability problems upon the high temperature storage conditions typical of a military user environment, as also upon long-term operation.